The present invention relates to synchronous reluctance motor drive control.
The synchronous reluctance motor has been recognized as the simplest form of polyphase synchronous motor (see "Synchronous Reluctance Motors" by P. B. Greenwood in Electrical Review, Mar. 22, 1968, pp. 432-434; see also P. J. Lawrenson and L. A. Agu "Theory and Performance of Polyphase Reluctance Machines" in Proceedings IEEE 1964 III (8), p. 1435).
Reluctance motors have simple and robust rotors which run at synchronous speed. They, however, have no starting torque and, when they are excited from the mains, it was found necessary to include a squirrel cage in the rotor to allow starting on line. In order to include a squirrel cage in the rotor it is necessary to change the optimum rotor geometry so as to allow sufficient room for the conducting material of the cage bars. This has resulted in degrading the synchronous performance of reluctance motors and increasing the cost of constructing the motor. The synchronous, synchronizing, asynchronous and starting performance characteristics of a reluctance motor (fed from the mains) depend upon the rotor design. Therefore, the design of a reluctance motor (fed from the mains) is a delicate compromise between conflicting requirements imposed on the rotor geometry to achieve an acceptable overall performance. This has been considered a drawback of reluctance motors.
The reluctance motor will develop small oscillations around the synchronous speed when excited from a variable frequency source (e.g., inverter), unless using feedback control. It can operate in an open loop up to a load angle .delta. around 45.degree.. It was found, however, that when a reluctance motor is excited from a variable frequency source and feedback control is utilized for stabilization, there is no need for cage bars. This was found to improve the efficiency-power factor product and to simplify significantly the rotor construction of the reluctance motor. Cageless reluctance motors have been built and tested with an appreciable improvement in performance and with reduction in the cost. The motor starts at zero frequency and the frequency is increased up to rated speed as the motor remains synchronized by feedback control. This has the important consequence of eliminating the use of cage bars in the rotor, the motor starting smoothly from zero speed. Furthermore, smooth starting of the motor eliminates the inrush of current when starting with a motor fed from the mains. This simplifies the control circuitry and reduces the size of the inverter. It is also highly desirable since there is no disturbance caused to the common grid. Furthermore, the elimination of cage bars eliminates what is a main drawback in the design of a reluctance motor.
Also, more efficient motors are being designed today. Cageless reluctance motors with a better efficiency (due to reduced losses) are superior because of higher power factor and larger output power than conventional reluctance motors. When the efficiency-power factor product is appreciably improved, since this product is inversely proportion to the inverter size, the inverter size and cost are consequently reduced. In conclusion, a cageless reluctance motor has better efficiency, power factor, starting performance, lower cost, and stable operation due to feedback control.
It has been shown that maximum efficiency, output power and power factor occur at a load angle between 45.degree. and 60.degree., these angles lying in the unstable operating region of the reluctance motor, therefore requiring feedback control for stable and efficient operating. See: "Steady-State Performance Characteristics Of Linear Reluctance Motors" by A. M. El-Antably, J. D. Edwards, G. Williams, P. Lindon and P. D. Luke in Magnetics, Vol. 15, No. 6, November 1979, IEEE. As a result of this rapid advancement in solid-state variable frequency sources, and due to the new developments (cageless rotors and feedback control), it is now possible to design reluctance motors which are comparable in size to, and of lower cost than, induction motors of the same rating. The distinct advantages of cageless reluctance motors are that they are cheaper than induction motors of comparable size, and more suitable for variable-speed drives. The reluctance motor has become a serious candidate to replace the induction motor for variable-speed drive applications.
Feedback control permits a stable operation of the reluctance machine at load angles greater than 45.degree. at which motor torque, power factor and efficiency are maximum. Feedback control also results in the elimination of the cage bars from the rotor, which improves efficiency and reduces cost. The present invention aims at a feedback control strategy for reluctance motors to optimize its performance and simplify its control.
The control strategy according to the present invention allows to operate a cageless reluctance motor as an energy efficient motor by control depending only upon measuring the terminal voltages and currents. Thus, there is no need for transducers to be mounted on the rotor shaft. This reduces cost and is advantageous in industrial applications when there is no access to the shaft. This feature combines with the fact that a reluctance motor is easier to control than an induction motor, thus, more suitable for variable speed motor drives.